Photoreceptor light adaptation models: an evaluation.

Two current descriptions of photoreceptor responses in the presence of adapting background illumination are evaluated. Both descriptions are based on the Michaelis-Menten equation V/Vm = In/(In + sigma n), but they use this equation in different ways: (1) to describe increment responses to light increments on a background (the "increment-light model") and (2) to describe the total receptor response as a function of total incident light intensity, i.e. increment plus background, (the "incident-light model"). It is shown that the incident-light model gives the best and most comprehensive description of photoreceptor responses. This conclusion is based on theoretical considerations and is supported with examples using rod and cone response data taken from the literature.